This research is in part motivated by the very large discrepancy between the assumptions of the demographic models addressing the question of the effect of immigration on population aging and the empirical evidence about the migration processes of the US's single largest immigrant- contributing country, Mexico. The nature of the demographic models is that they assume that immigrants settle in the receiving country. The available empirical evidence with regard to Mexico is that large numbers of immigrants do not settle in the US, and instead return to Mexico. This evidence, however, does not come from nationally representative data sources. The study aims first to improve understanding of the age and gender patterns of US-Mexico migration in both directions, and second to improve understanding of their consequences for Mexican-born immigrants' patterns of aging in the US versus in Mexico. The major methodological innovation of the study is its use of data sources from both the US and Mexico together to estimate both rates of immigration to the US and rates of return emigration to Mexico. These data sources are the 1990 and 2000 censuses and the large-scale household surveys conducted in both the US and Mexico from the 1990s through to the mid-2000s. With the migration rates estimated by combining these data sources as input parameters, a life table model is used to derive Mexican-born cohorts' ages of residence in the US and in Mexico over the lifetime. This directly addresses the question of to what extent Mexican immigrants are likely to age in the US versus in Mexico. The extent to which more recent migrant streams from Mexico are becoming increasingly balanced by gender, and consequently more likely to remain in the US into older ages, is evaluated by comparing the patterns of migration by age and gender in the late 1980s, late 1990s, and through to the mid-2000s. The selectivity of migrants by educational attainment is estimated in both immigrant and emigrant flows. The educational selectivity of Mexican-born immigrants who remain in the US into old age is then analyzed as a function of both selectivity in the process of immigration and selectivity in the process of return migration. The study is a first step towards developing a broader understanding of the impact of immigration on the age, sex, and socio-economic structure of the US population, and thus also on the US's ability to support an older population with greater health needs. [unreadable] [unreadable] [unreadable] [unreadable]